As I am currently flying for an airline whose first language is not English, I'd like to make the following observation. Not all pilots have an excellent grasp of English, especially at busy times when the clearance is delivered rather quickly. So please try to make allowances with foreign registered airlines, when issuing such clearances - they do not want to 'get it wrong' but sometimes their command of the language requires them to mentally translate it into their own language before replying. The clearance you mention is perhaps the most confusing of all, especially as it is expected to be read back in full under pressure. Whatever the conditional clearance, there will always be room for error.
Recently I was given the following clearance "XXX123 behind the landing YYY 737, line up runway 00L behind" - is this not the correct terminology ( I don't have access to any R/T phraseology literature at the moment). I'm sure that you appreciate that acrooss the 'pond' verbage is often different!!